Cordless or wireless communications systems are widely used to provide users with mobile communications. In general, the term cordless can refer to any form of air wave transmission using a set of radio frequencies. Conventional implementations of cordless systems, for example, include both public cordless systems and inbuilding cordless systems. In public systems, there are numerous service providers that allow users to make and receive calls virtually anywhere within a service area. Such service providers offer solutions based on a number of different technologies and standards. Typically, the service providers have purchased a license from the federal government (i.e., Federal Communications Commission) to use a specific portion of the radio spectrum within specific markets.
In contrast to public systems, in-building cordless systems can avoid the costs of radio spectrum licenses by using unlicensed radio frequencies. In-building systems typically have a common configuration or topography in that there is a radio exchange that is adjunct to or integrated with a private branch exchange (PBX). Base stations (or fixed parts) are equipped with radio antennas that connect to the radio exchange. The base stations also transmit radio signals to and receive radio signals from cordless handsets (portable parts) within a limited range.
With respect to unlicensed radio frequencies, cordless systems often use the ISM (Industrial, Scientific and Medical) band. In the United States, ISM based devices are regulated by and must follow Federal Communications Commission (FCC) guidelines. In general, FCC guidelines implement restrictions on the use of frequencies within the ISM band. For example, devices are allowed to communicate at a particular frequency only with a defined bandwidth for a defined period of time and with a defined signal power level. Since the ISM band is unlicensed, it is used by many vendors for various types of cordless devices (e.g., medical monitoring devices, wireless LANs, printers, speakers, security systems and in-building cordless systems). Consequently, radio frequency (RF) interference can be a significant problem with using the ISM band.
For a cordless telephone system using the ISM band, the FCC restrictions produce a need to implement a frequency hopping scheme to ensure that the cordless system does not violate restrictions on the use of frequencies within the ISM band. Frequency hopping can achieve this by allowing base stations and handsets to move in sync from frequency to frequency in the time domain. Further, when implementing such a frequency hopping scheme, there is a need to implement a scheme for avoiding bad channels or frequencies due to RF interference and other problems.
Such frequency hopping can be problematic, however. For example, if the base station and the cordless handset are not synchronized, they will not hop together and thus transmissions will not be successful. Additionally, if an in-building cordless system includes more than one base station, the two base stations can interfere with each other if they hop to the same frequencies during the same time period. Additionally, it can be difficult to modify the hop sequences.